Midfoot arthrodesis, which involves the fusion of bones in the medial column, is a common treatment for a number of foot deformities and conditions. Conventional lag screws are a common surgical implant device employed for midfoot athrodesis. A surgical lag screw generally comprises a head and a shaft which is partially threaded at the end which is opposite the head. A portion of the shaft immediately adjacent the head is generally smooth and without threads. For purposes of bone fusion, a lag screw generally works by engaging the threaded portion of the shaft into a bone and, in combination with the head of the screw, compressing the bone in which the threads are imbedded against a neighboring bone through which the smooth portion of the screw shaft passes. The compressive forces between the bones is referred to as the lag effect. Lag screws may be cannulated with a hollow center. This permits alignment with a guide wire or guide pin.
IO Fix (trademark of Extremity Medical) is a surgical implant generally comprising a pair of screws, with a first screw imbedded in a single bone and a second screw imbedded in two adjacent bone structures, supplying a lag effect therebetween. The first screw gives support to the second screw and changes the distribution of compression forces.
The Midfoot Fusion Bolt (by DePuy Synthes) is an intramedullary implant that can be used to fuse the medial metatarsocuneiform, naviculocuneiform, and talonavicular joints. The implant generally comprises a solid bolt which is inserted through the upper center of the first metatarsal head close to the dorsal cortex.
US Patent App. Pub. No. 2010/0256639 A1 describes an intramedullary fixation assembly having a first member and a second member. After insertion into the bone, the first and second members are held in place by pairs of retaining screws which pass through the first and second members.
In spite of existing solutions, post-operative problems still exist for some patients after a midfoot arthrodesis procedure. The main problems with diabetic Charcot medial column fusion are non-rigid fixation and wound infection with prominent hardware. Furthermore, many existing solutions are undesirable and cost prohibitive. Despite the surgery's intent to rigidly fix the bones for which fusion is desired, small and potentially deleterious movement does sometimes still occur between the bones, inhibiting or preventing the body's induced fusion by ossification. This arises from limitations of the existing surgical implants. Traditional lag screws, for example, may sometimes rotate or shift and loosen the compressive forces between the bones. In addition, many of the existing solutions do not allow for fusion of more than two bones. As a result, it is not uncommon for several implants, e.g. several conventional lag screws, to be needed for the fusion of three or more bones in the midfoot.